I think you need to break down the individual systems on boats to figure out what is going to hold value and what is going to lose value. I also think we've entered another "new generation" of boats in the last couple of years when the independent systems began to become integrated into a central control/computer. The vectoring rudders/skegs, surf gates, speed control with ballast integration, etc are all going to detract from the value of used boats in a few years. These sytems are going to be great as long as they are under warranty, but once OEM support/parts dries up -- you're going to be proper SCREWED.
It's not a problem that will be independent to the marine industry, however -- so there is hope. All industries are now fully saturated with integrated electronics. As these systems begin to break down and fail, I suspect that niche businesses will blossom that can repair/replace these systems will emerge. There will certainly be a demand for it.
Certain systems are going to be fine, though. All of the engine and engine management components have been used so extensively that they will be continued to be available and supported. Are they more complicated? Of course. But people will figure out ways to work on these systems and keep them going.
Compare the market with that of cars -- there isn't a huge pricing gap between old "simple" cars and newer cars with complex, integrated systems. People aren't junking their new cars because older cars are easier to work on. The trade off of better convenience, economy, function and comfort comes with having more stuff to break and repair bills sometimes being higher. It's just the way things are now.